1. Field of the Invention
The present invention relates to a predistortion type linearizer for compensating for the nonlinear gain and phase deviation characteristics of a nonlinear amplifier.
2. Description of the Related Art
Recently, digital cellular communication systems have been developed that demand linear power amplifiers. In order to satisfy the high linear characteristic requirement, a linear power amplifier is constructed by a nonlinear amplifier and a predistortion type linearizer for compensating for the gain and phase deviation characteristics of the nonlinear amplifier. That is, the gain and phase deviation characteristics of the predistortion type linearizer are opposite to those of the nonlinear amplifier.
In a prior art predistortion type linearizer including a field effect transistor (FET), an input matching circuit connected to the drain of the FET for receiving an input signal, an output matching circuit connected to the source of the FET for outputting an output signal, and an inductor having a first terminal connected to the gate of the FET and a second terminal for receiving a first control voltage, a constant impedance circuit is connected to the second terminal of the inductor. Thus, independent control of the gain and phase deviation characteristics of the predistortion type linearizer are achieved by changing the value of the inductor and the value of the control voltage (see JP-A-2000-357926). This will be explained later in detail.
In the above-described prior art predistortion type linearizer, however, after the inductor is fixed to the predistortion type linearizer, it is impossible to independently change the gain and phase deviation characteristics by the control voltage.
It is an object of the present invention to provide a predistortion type linearizer capable of easily adjusting the gain and phase deviation characteristics independently.
According to the present invention, in a predistortion type linearizer including a FET, an input matching circuit connected to the drain of the FET for receiving an input signal, an output matching circuit connected to the source of the FET for outputting an output signal, and an inductor having a first terminal connected to the gate of the FET and a second terminal for receiving a first control voltage, a variable impedance circuit is connected to the second terminal of the inductor, and the impedance of the variable impedance circuit is adjusted by a second control voltage. Thus, the gain and phase deviation characteristics can be adjusted by the first and second control voltages without adjusting the value of the inductor.